1. Field of the Invention
The present invention is directed to tuning volume holograms and, more particularly, to a method of broadening the bandwidth and/or shifting the transmission and reflection frequencies of volume holograms and stabilizing the resulting film from further change and damage.
2. Description of the Related Art
Volume holograms are characterized by three dimensional Bragg planes created throughout the volume of the recording medium. The recording medium when developed includes a very delicate microstructure which is easily damaged by abrasion, pressure, moisture and organic solvents. The holograms are typically used as transmissive or reflective filters and it is difficult to produce a filter that operates at wavelengths longer than the recording wavelength. Once created, the center frequency of the hologram tends to drift over time toward the blue end of the visible light spectrum. This drift is caused by the gradual collapse of the microstructure of the Bragg planes. It is desirable to make these structures rigid, resistant to abrasion and resistant to frequency shift.
The prior art technique for protecting the hologram film is to encapsulate the hologram in glass or plastic to protect the film from abrasion, solvents and moisture. This technique does not prevent center frequency shift and the hologram continues to drift in playback color.